1. Field of the Invention
The present invention relates in general to a method and an apparatus for measuring the hysteresis and drift behaviors of an ISFET. In particular, the present invention relates to a method and an apparatus for measuring the hysteresis and drift behaviors of an ISFET that uses the hydrogenated amorphous silicon as a sensing film.
2. Description of the Related Art
The ion sensitive field effect transistor (ISFET) is constructed by substituting a sensing film for the metal gate on the gate oxide of a traditional MOSFET. When the ISFET is dipped into a solution, the interfacial potential between the sensing film and the solution will influence the semiconductor surface since only an extremely thin dielectric (that is, the gate oxide) separates the sensing film and the semiconductor surface. This influences the charge density in the inversion layer of the semiconductor surface, and thereby modulates the channel current passing through the ISFET. Therefore, by utilizing this characteristic, the pH value or other ion concentration in the solution can be deduced from the measurement of source/drain current and the gate voltage of the ISFET. Furthermore, the potential difference on the interface between the sensing film and the solution is in relation to the ion activity in the solution. The hydrogen ion activity in the solution can be measured by using different channel currents caused by different interfacial potential differences in various solutions with different hydrogen ion activity.
The patents, which are related to the formation of the ISFET or the measurement of the ISFET, are listed hereinafter.
(1) U.S. Pat. No. 5,414,284 discloses a method of fabricating an ISFET and an ESD protective circuit on the same silicon substrate, wherein a capacitor is utilized as an interface between the protective circuit and a sample solution to isolate the D.C. leakage current.
(2) U.S. Pat. No. 5,911,873 discloses a method and an apparatus for determining and controlling the zero potential point by measuring currents and gate voltages. Also, an apparatus for measuring the ion concentration in the solution is provided with an ISFET, a reference electrode, a control circuit, a memory, a measuring circuit and a diagnostic circuit. The control circuit makes the ISFET operate under a drain-source currents and plurality of continuous drain current and gate voltages. The measuring circuit and diagnostic circuit measure the ion concentration and the device characteristics. Finally, the measuring results are recorded in the memories.
(3) U.S. Pat. No. 5,833,824 discloses an ISFET sensor for sensing the ion activity in the solution includes a substrate and an ISFET semiconductor die. The substrate has the front surface exposed to the solution, a back surface opposites to the front surface and an aperture extending between the front and back surface. A sensing film of the ISFET is mounted to the back surface such that the gate region is exposed to the solution through the aperture.
(4) U.S. Pat. No. 5,602,467 discloses a method of measuring the ion concentration in the solution by using two differential amplified circuits that are composed of four ISFETs and four FETs. The circuit configuration shows the difference between two ISFETs. Also, the technical errors, the variation in parameters during operation and the effects of environment can be ignored. The circuit comprises two amplifiers, and the amplifiers comprise two ISFETs and a FETs that are different or identical. The output of the first amplifier occurs the difference of the mean value of the ISFET threshold voltage and the FET threshold voltage, and the output of the second amplifier occurs the difference of the two ISFET threshold voltages.
(5) U.S. Pat. No. 5,387,328 provides a method of measuring the glucose concentration by fixing the enzyme on a sensing film and using platinum (Pt) as a reference electrode. The sensor has a Pt electrode being capable of sensing all biological substances which generate H2O2 in enzyme reaction, whereby having the high sensitivity and rapid time.
(6) U.S. Pat. No. 5,350,701 provides a method of measuring the content of alkaline-group metals, especially the content of the calcium ions, by utilizing chemosynthesis phosphide group as the sensing film on a gate of an ISFET.
(7) U.S. Pat. No. 5,319,226 discloses a Ta2O5 sensing film deposited by using a radio frequency sputtering method on an ISFET, wherein the ISFET consists of a Ta2O5/Si3N4/SiO2 structure. In this case, a Ta2O5 film of the thickness between 400xcx9c500  xc3x85 is formed on a Si3N4/SiO2 dielectric, and the Ta2O5 film outside the gate is removed by a lift-off process utilizing a positive PR film. The Ta2O5 gate ph-ISFET according to the present invention has higher sensitivity and more stable, while the productivity and stability thereof are greatly improved affecting a whole wafer process.
(8) U.S. Pat. No. 5,314,833 discloses a method which depositing a silicon film on a GaAs substrate and doping arsenic/phosphorous ions into the silicon film to fabricate the gate with lower resistance. This can reduce the thermal effect on the device, and improve the operation characteristics.
(9) U.S. Pat. No. 4,812,220 provides an ISFET made by fixing the enzyme on the sensing film for measuring the concentration of the amino acid in food. The enzyme sensor is miniaturization, and can accurately measure the concentration even when the content of the amino acid is small.
(10) U.S. Pat. No. 4,657,658 provides a differential pair modular system composed of a MOS transistor and an ISFET. Also, a semiconductor IC circuit for sensing surrounding physical/chemical characteristics is provided, which comprises a pair of semiconductor devices similar to a geometric and physical structure. One semiconductor device is employed to sense the characteristics. The other semiconductor device that is insensitive connected to another pair of semiconductor devices to form a feedback loop together with a differential amplifier.
However, in the ISFET applications, many factors such as the hysteresis and drift behaviors affect the accuracy of the measuring results. With reference to the hysteresis behavior, it is related to the change in the pH value of the solution (such as pHxxe2x86x92pHyxe2x86x92pHxxe2x86x92pHzxe2x86x92pHx) and the corresponding change in the output voltage of the ISFET (such as Vox1xe2x86x92Voyxe2x86x92Vox2xe2x86x92Vozxe2x86x92Vox3). At the same pH value, the difference between the first output voltage and the final output voltage (such as Vox3xe2x88x92Vox1) is defined as the hysteresis value. As to the drift behavior, the drift value is defined as the change in the gate voltage per unit time on conditions that the source/drain current is stable and the temperature is constant. To improve the accuracy of the measuring results, measure the hysteresis value and the drift value to get the accurate output value of the ISFET by using a reverse compensation method after measuring all output values in an ordinary experiment.
Besides, the common ISFET has a sensing film made of Al2O3, Ta2O5 or Si3N4, the cost of which is extremely high and the production of which needs a long time. This product is not applicable to mass production, and cost thereof is much greater.
An object of the present invention is to provide a method of measuring the hysteresis value and the drift value of the a-Si:H ISFET so as to use the reverse compensation method to obtain the accurate output value.
Another object of the present invention is to provide an apparatus for measuring the hysteresis value and the drift value of the a-Si:H ISFET, wherein the process of forming the a-Si:H ISFET is easy, the cost is lower and applied to mass production.
To achieve the object of the present invention, a method of measuring the hysteresis value of an a-Si:H ISFET is provided, which comprises the steps of: (a1) using a constant voltage/current circuit to fix the drain/source current and the drain/source voltage of the a-Si:H ISFET; (a2) dipping the sensing film into a buffer solution; (a3) recording the gate/source output voltage of the a-Si:H ISFET by using a voltage-time recorder; and (a4) exchanging the pH value of the buffer solution and then repeating the steps of (a1)xcx9c(a3) so as to measure the hysteresis value of the a-Si:H ISFET.
To achieve another object of the present invention, a method of measuring the drift value of an a-Si:H ISFET is provided, which comprises the steps of: (b1) sing a constant voltage/current circuit to fix the drain/source current and the drain/source voltage of the a-Si:H ISFET; (b2) dipping the sensing film into a buffer solution; and (b3) recording the gate/source output voltage of the a-Si:H ISFET during a constant period by using a voltage-time recorder so as to measure the drift value of the a-Si:H ISFET.
To achieve another object of the present invention, an apparatus for measuring the hysteresis value and the drift value is provided, which comprises an a-Si:H ISFET that is formed on a semiconductor substrate and comprises source/drain regions which isolate each other and a sensing film made of the hydrogenated amorphous silicon; a buffer solution for touching the a-Si:H ISFET; a light-isolation container for isolating light and carrying the devices; a heater for heating the buffer solution; a constant current/voltage circuit connected to the source and drain of the a-Si:H ISFET; a current/voltage measuring device connected to the constant current/voltage circuit; and a voltage-time recorder connected to the constant current/voltage circuit.
This and other objectives of the present invention will no doubt become obvious to those of ordinary skills in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.